U.S. patent application number 12/110563 was filed with the patent office on 2008-10-30 for orthopedic fixation device with zero backlash and adjustable compliance, and process for adjusting same.
Invention is credited to John Peter Karidis.
Application Number | 20080269741 12/110563 |
Document ID | / |
Family ID | 39887871 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080269741 |
Kind Code |
A1 |
Karidis; John Peter |
October 30, 2008 |
ORTHOPEDIC FIXATION DEVICE WITH ZERO BACKLASH AND ADJUSTABLE
COMPLIANCE, AND PROCESS FOR ADJUSTING SAME
Abstract
An orthopedic fixator for positioning a first element relative
to a second element with precision and with controlled compliance
which can be adjusted during the healing process. One embodiment
comprises a first frame for attachment to the first element, a
second frame attached to the first frame through a plurality of
adjustable effective length struts, and a third frame for
attachment to the second element, wherein the third frame is
compliantly attached to the second frame. A preferred embodiment
comprises adjustable length preload elements to apply
unidirectional forces between the first and second frames so as
preload the adjustable effective length struts and substantially
reduce the positional tolerance. An alternative embodiment
comprises adjustable spring elements allowing the compliance of the
attachment of the third frame to the second frame to be adjusted at
various points in the healing process.
Inventors: |
Karidis; John Peter;
(Ossining, NY) |
Correspondence
Address: |
MYERS WOLIN, LLC
100 HEADQUARTERS PLAZA, North Tower, 6th Floor
MORRISTOWN
NJ
07960-6834
US
|
Family ID: |
39887871 |
Appl. No.: |
12/110563 |
Filed: |
April 28, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60926597 |
Apr 28, 2007 |
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Current U.S.
Class: |
606/56 ; 606/246;
606/54 |
Current CPC
Class: |
A61B 17/62 20130101 |
Class at
Publication: |
606/56 ; 606/246;
606/54 |
International
Class: |
A61B 17/00 20060101
A61B017/00; A61B 17/70 20060101 A61B017/70 |
Claims
1. An orthopedic fixator comprising: a) a first frame and a second
frame; b) a plurality of adjustable effective length struts
connecting the first frame to the second frame so as to fix the
position of said second frame relative to said first frame to
within a positional tolerance; and c) at least one preload element
disposed between the first and second frames for preloading at
least one of the adjustable effective length struts to
substantially reduce the positional tolerance.
2. The orthopedic fixator of claim 1, wherein the at least one
preload element further comprises a plurality of adjustable length
flexible tension and/or compression elements.
3. The orthopedic fixator of claim 2, wherein the adjustable length
compression element further comprises the adjustable effective
length strut modified to resist shortening beyond an adjustable
length and to allow lengthening beyond said adjustable length.
4. The orthopedic fixator of claim 1, wherein the at least one
preloading element primarily preloads only a pair of adjacent
effective length struts of the plurality.
5. The orthopedic fixator of claim 1, wherein one end of the at
least one preload element is positioned on at least one of the
frames near the mid-point position between a pair of adjustable
effective length struts.
6. The orthopedic fixator of claim 1, further comprising at least
one preload element for every two adjustable effective length
struts.
7. The orthopedic fixator of claim 1, wherein the actuation of the
at least one preload element constrains independent adjustment of
adjacent adjustable effective length struts while not substantially
constraining independent adjustment of non-adjacent adjustable
effective length struts.
8. A method of preloading an orthopedic fixator comprising a first
frame, a second frame, and a plurality of adjustable effective
length struts connecting the first frame to the second frame so as
to fix the position of said second frame relative to said first
frame to within a positional tolerance, the method comprising
providing at least one preload element between the first and second
frames for preloading at least one of the adjustable effective
length struts to substantially reduce the positional tolerance.
9. The method of claim 8, wherein the actuation of the at least one
preload element constrains independent adjustment of adjacent
adjustable effective length struts while not substantially
constraining independent adjustment of non-adjacent adjustable
effective length struts.
10. The method of claim 8, further comprising a method of adjusting
the relative position of the two frames under preload from the at
least one preload element, the method of adjusting comprising: a)
substantially reducing a preload force generated by the at least
one preload element; b) adjusting a length of some or all of said
adjustable effective length struts; and c) substantially increasing
the preload force generated by the at least one preload element to
substantially reduce positional tolerances resulting from
mechanical tolerances in the construction or assembly of the
adjustable effective length struts or the orthopedic fixator.
11. The method of claim 10, wherein the length of only the
adjustable effective length struts that are adjacent the at least
one preload element are adjustable when the preload force is
substantially reduced from the at least one preload element.
12. The method of claim 10, further comprising adjusting the length
of adjustable effective length struts in pairs for each preload
element having a substantially reduced preload force.
13. The method of claim 10, further comprising a plurality of
preload elements and wherein the method of adjusting further
comprises repeating the sequence of steps (a)-(c) with respect to
one preload element at a time.
14. An orthopedic fixator comprising: a) a first frame, a second
frame, and a plurality of struts connecting the first frame to the
second frame to fix the position of the first and second frames to
within a positional tolerance; b) a third frame connected to the
first frame; and c) a compliant attachment defined between the
first and third frames.
15. The orthopedic fixator of claim 14, wherein the first and third
frames are concentric.
16. The orthopedic fixator of claim 14, further comprising at least
one preload element disposed between the first and second frames
for preloading at least one of the struts to substantially reduce
the positional tolerance.
17. The orthopedic fixator of claim 14, further comprising an
adjustable compliant attachment defined between the first and third
frames.
18. The orthopedic fixator of claim 14, further comprising at least
one spring plate attached at a first location to one of said first
frame and said third frame and attached at a second location to the
other of said first frame and said third frame.
19. The orthopedic fixator of claim 18, wherein the second location
further comprises a radial extension of the third frame.
20. The orthopedic fixator of claim 19, wherein the first and
second locations are circumferentially aligned.
21. The orthopedic fixator of claim 18, wherein the at least one
spring plate is attached at the second location by at least one
positionable clamp, wherein the position of the at least one
positionable clamp can be varied to change a distance between the
first and second locations.
22. The orthopedic fixator of claim 21, wherein the at least one
positionable clamp is movable from a first position closest to the
first location, which results in minimum compliance, to a second
position spaced from the first location, which results in increased
compliance between the first and third frames.
23. The orthopedic fixator of claim 22, further comprising a
plurality of positional clamps.
24. The orthopedic fixator of claim 18, further comprising at least
a pair of spring plates positioned on opposite sides of the first
frame to create at least one parallel flexure support.
25. The orthopedic fixator of claim 14, further comprising physical
stops for limiting a displacement of the third frame relative to
the first frame to a maximum distance, wherein the maximum distance
is independent of the degree of compliance provided by the
compliant attachment between the first and third frames.
26. The orthopedic fixator of claim 25, wherein the maximum
displacement of the third frame relative to the first frame in a
direction towards the second frame is different than the maximum
displacement of the third frame relative to the first frame in the
direction away from the second frame.
27. The orthopedic fixator of clam 14, wherein the compliant
attachment provides a nonlinear force-displacement response whereby
the displacement of the third frame relative to the first frame
increases by less than a factor of two when an external force
applied to the compliant attachment increases by a factor of
two.
28. The orthopedic fixator of claim 14, wherein one of said first
frame and said third frame further comprises angular openings and
the other of said first frame and said third frame comprises radial
extensions that extend within the angular openings to prevent the
first and third frames from rotating relative to each other.
29. A method for adjusting the compliance of an orthopedic fixator
comprising a plurality of adjustable compliance attachments
positioned between a first fixator element and second fixator
element, wherein: a) the adjustable compliance attachments further
comprise a non-adjustable state and an adjustable state; and
wherein b) at least one of the adjustable compliance attachments of
the plurality remains in the non-adjustable state at all times.
30. The method of claim 29, wherein the adjustable state comprises
an un-clamped state wherein the position of the adjustable
compliance attachment can be changed, and further wherein the
position of the adjustable compliance attachment determines the
compliance of the orthopedic fixator.
31. The method of claim 30, further comprising: c) reducing a clamp
force of at least one of the adjustable compliance attachments; d)
re-positioning at least one of the adjustable compliance
attachments; e) increasing the clamp force of the re-positioned
adjustable compliance attachments; and f) repeating the above steps
for other adjustable compliance attachments until all desired
adjustments are completed.
32. An orthopedic fixator comprising: a) a first frame adjustably
attached to a second frame; b) a third frame compliantly attached
to the first frame; c) multiple adjustable compliant attachments of
the first frame to the third frame, the compliant attachments being
independently adjustable to create a remote axis of rotation for
the third frame.
33. The orthopedic fixator of claim 32, wherein the first and third
frames are concentric.
34. The orthopedic fixator of claim 32, further comprising stops
for limiting the displacement of the third frame relative to the
first frame to a maximum distance, wherein the maximum distance is
independent of the degree of compliance provided by the compliant
attachment between the first and third frames.
35. The orthopedic fixator of claim 34, wherein the maximum
displacement of the third frame relative to the first frame in a
direction towards the second frame is different than the maximum
displacement of the third frame relative to the first frame in the
direction away from the second frame.
36. The orthopedic fixator of claim 32, wherein the compliant
attachments provide a nonlinear force-displacement response whereby
the displacement of the third frame relative to the first frame
increases by less than a factor of two when an external force
applied to the compliant attachments increases by a factor of
two.
37. A process for orthopedic fixation of two skeletal elements
during healing, comprising: a) fixing a position of a first
skeletal element relative to a second skeletal element using an
orthopedic fixator with adjustable compliance; b) adjusting the
position and the compliance of the orthopedic fixator to minimize
motion of the skeletal elements during a first phase of healing;
and c) increasing the adjustable compliance of the fixator in at
least one direction during a second phase of healing.
38. The process of claim 37, further comprising further increasing
the adjustable compliance of the fixator in at least one direction
after the second phase of healing.
39. The process of claim 37, wherein the compliance of the
orthopedic fixator is non-linear and the relative motion of the
skeletal elements increases by substantially less than a factor of
two when the external applied force on the skeletal elements is
increased by a factor of two.
40. The process of claim 37, further comprising limiting the
maximum motion of the first skeletal element relative to the second
skeletal element.
41. The process of claim 40, wherein the maximum motion of the
first skeletal element towards the second skeletal element is
different than the maximum motion of the first skeletal element
away from the second skeletal element.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Application
60/926,597, filed Apr. 28, 2007, the contents of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to orthopedic fixation
systems and more specifically to an improved external fixator with
zero backlash and continuously adjustable compliance, and process
for adjusting same.
BACKGROUND
[0003] For centuries, external splints of various forms have been
used to provide skeletal support to injured or healing limbs. For
decades, various forms of external fixation have been used by
orthopedic surgeons to support bones that are healing from
traumatic fracture, or that are healing from reconstruction surgery
intended to correct deformities by repositioning, lengthening or
shortening various bone segments. These external fixation devices
can use single shafts attached to bone elements by half-pins, or
circular arcs or rings which can be attached to bone elements by
half-pins or by tensioned wires that pass all the way through the
limb (known as the Ilizarov technique).
[0004] In general, the goal of these external fixators is to
maintain the relative position of two bone segments during healing.
The desired relative position may be fixed (as in the case of
simple trauma healing) or variable (as in the case of gradual bone
lengthening or deformity correction). Also, the desired stiffness
of the system may be very high in some cases, such as the initial
healing phase of unstable oblique fractures, and may be lower in
other cases where some external load sharing by the healing bone is
desired. Many types of external fixators have been developed. One
of the most sophisticated, adaptable, and easily adjustable of
these is the Taylor Spatial Frame, developed by Harold S. Taylor,
J. Charles Taylor, et al, and marketed by Smith & Nephew,
Inc.
[0005] Taylor et al., U.S. Pat. No. 5,702,389 describes a variety
of fixator types, and discloses a ring-type external fixator based
on a six-degree-of-freedom "Stewart Platform." In this design, six
adjustable-length struts are used to connect a first base member
for mounting to a first bone element to a second base member for
mounting to a second bone element. Spherical joints which are
common to the ends of two different struts are used to pivotably
mount the struts to the base members.
[0006] Taylor et al., U.S. Pat. No. 6,030,386 retains the same
basic structure of six adjustable-length struts connected between
two base members, but replaces the spherical end joints with a
combination of a rotating joint plus two pivoting joints which all
share a common axis so as to allow independent rotation of each
strut about its axis, in addition to the required two axis of
pivoting required at the mounting ends.
[0007] Taylor et al., U.S. Pat. No. 5,891,143 discloses a
particular design for a family of base members having different
diameters, whereby all of the family members contain a
circumferential array of holes with fixed separation, and whose
total number is divisible by three. These holes are designed to
support mounting blocks holding half-pins, Ilizarov wires, or other
hardware which is attached to the bone elements.
[0008] The Taylor Spatial Frame provides good range of motion and
adjustability, but each of a large number of mechanical joints and
threaded parts in each strut adds some inevitable amount of
mechanical clearance. The sum of all of these small mechanical
tolerances results in a non-negligible amount of mechanical "play"
in the system. The possible variations in the precise positioning
and the distance between the strut ends create kinematic
uncertainty that substantially limits the positional accuracy and
precision by which the base members are held.
[0009] More specifically, the design of the Taylor Spatial Frame,
as shipped commercially and as taught in U.S. Pat. No. 6,030,386,
introduces small but necessary mechanical tolerances at several
locations on each end of each adjustable length strut, including
but not limited to radial and axial tolerances between the shoulder
screw and the base member, radial and axial tolerances at each of
the two pivot joints at each end, axial thread clearances between
the threaded rod and the adjustment nut, and axial clearances
between the internal retaining ring and the corresponding retential
grooves in both the adjustment nut and non-threaded portion of the
strut.
[0010] The end result of these cumulative tolerances is mechanical
uncertainty (or "play") of somewhere on the order of 1 mm in any
direction, and on the order of 1 degree in rotation about multiple
axes. While some units may be substantially more accurate than
this, the practical manufacturing tolerances that can be achieved
on this many parts, together with the kinematic magnification of
errors that can occur in some configurations, means that there may
always be some perceptible level of clearance.
[0011] This mechanical clearance creates two deficiencies. The
first deficiency is the inability of the structure to precisely and
rigidly maintain the relative position of the base members. While
the overall rigidity of the frame in high for large applied
motions, the rigidity for small motions is nearly zero. This can
result in unwanted bone motion and the unwanted transmission of
external loads to the bones during certain healing phases.
Furthermore, this mechanical clearance can result in the generation
of acoustic noise in response to applied loads. The acoustic noise
can potentially be noticeable enough to attract unwanted attention
and/or to disturb the sleep.
[0012] An additional deficiency of the current art is the inability
to controllably adjust the stiffness (or its inverse, the
compliance) of the structure. The compliance of the external
fixator determines the degree to which external loads are carried
by the fixator frame itself, and the degree to which they are
transmitted to and carried by the bone. Generally, it is believed
that bone fracture healing and bone regeneration is affected by the
level of mechanical rigidity that is provided by fixation devices
during the healing process. Furthermore, it is generally believed
that the optimal level of fixation rigidity varies during the
fracture healing process, with maximum rigidity (i.e., minimum
compliance) generally being most appropriate during the initial
primary healing or callous formation phases, and with progressively
lower rigidity (i.e., higher compliance) being most appropriate
during the later callous remodeling phase when load sharing between
the bone and the frame is required.
[0013] In summary, many current orthopedic fixation devices, such
as the Taylor Spatial Frame, have manufacturing tolerances that
significantly limit the maximum stiffness that can be achieved for
small displacements, and no known available fixation devices
provide a means for controllably adjusting the stiffness to a lower
level (i.e., increasing the compliance of the fixator) if desired
during later healing stages.
SUMMARY OF THE INVENTION
[0014] An orthopedic fixation device is provided that, in one
embodiment, uses adjustable preloading elements to eliminate
backlash and provide for more precise and stable fixation. Such
device has several advantages including, but not limited to,
reducing patient discomfort resulting from undesired motion and
generating less mechanical noise. A further advantage is to provide
a device whose stiffness can be adjusted to improve bone healing
and patient comfort by providing a controllable level of
load-sharing between the frame and the supported bones.
[0015] Yet another advantage is to provide a device having
anisotropic stiffness so that axial motion has higher compliance
than motion in other degrees-of-freedom. Still yet another
advantage is to provide a device having the ability to adjust the
anisotropic stiffness characteristics to provide a combination of
linear motion and rotation in response to axial loading. Still yet
another advantage of an embodiment of the invention is to provide
an orthopedic fixation device having nonlinear force-deflection
characteristics and motion limit-stops, to allow load sharing at
low loads while limiting deflection at high loads.
[0016] In one embodiment of the invention, there is disclosed an
orthopedic fixator for positioning a first element relative to a
second element, said fixator comprising: a first frame for
attachment to the first element, a second frame for attachment to
the second element, a plurality of adjustable effective length
struts connecting the first frame to the second frame so as to fix
the position of the second frame relative to the first frame to
within a positional tolerance, and means for applying
unidirectional forces between the first and second frames so as
preload the adjustable effective length struts and substantially
reduce the positional tolerance.
[0017] In one embodiment, there is disclosed a process for
adjusting the precise relative position of two elements connected
by an orthopedic fixation assembly having a plurality of adjustable
effective length struts having at least one adjustable preload
element, comprising the steps of: substantially reducing the
preload force generated by the preload element, adjusting the
length of some or all of the adjustable effective length struts,
and substantially increasing the preload force generated by the
preload element to substantially reduce positional tolerances
resulting from mechanical tolerances in the construction or
assembly of the adjustable effective length struts or the
orthopedic fixation assembly.
[0018] In accordance with an alternative embodiment of the
invention, there is disclosed an orthopedic fixator for positioning
a first element relative to a second element, said fixator
comprising: a first frame for attachment to the first element, a
second frame adjustably attached to the first frame, a third frame
for attachment to the second element, and means for compliant
attachment of the second frame to the third frame. Furthermore, the
compliance of the attachment can be adjusted by changing the
position of slidable clamp elements along the length of a portion
of parallel spring plates. Additional aspects can include the use
of cantilevered spring elements attached at both ends so as to
provide non-linear force-deflection response, and mechanical
limit-stops that prevent excessive motion of the bone elements even
under high loading when the compliant structure is adjusted to
provide low stiffness (i.e., high compliance).
[0019] In accordance with an alternative embodiment of the
invention, there is disclosed an orthopedic fixator for positioning
a first element relative to a second element, said fixator
comprising: a first frame for attachment to the first element, a
second frame adjustably attached to the first frame, a third frame
for attachment to the second element, multiple adjustable compliant
attachments of the second frame to the third frame, and means for
independently adjusting the multiple adjustable compliant
attachments to create an effective axis of maximum compliance the
same as or different than the axis of maximum compliance of any of
the individual compliant attachments.
[0020] In accordance with an alternative embodiment of the
invention, there is disclosed an orthopedic fixator for positioning
a first element relative to a second element, said fixator
comprising: a first frame for attachment to the first element, a
second frame, a third frame for attachment to the second element, a
plurality of adjustable effective length struts connecting the
first frame to the second frame so as to fix the position of the
second frame relative to the first frame to within a positional
tolerance, means for applying unidirectional forces between the
first and second frames so as preload the adjustable effective
length struts and substantially reduce the positional tolerance,
and means for compliant attachment of the second frame to the third
frame.
[0021] In accordance with an alternative embodiment of the
invention, there is disclosed a process for orthopedic fixation of
two skeletal elements during healing, comprising the steps of:
fixing the position of a first skeletal element relative to a
second skeletal element using an orthopedic fixator with adjustable
compliance, adjusting the position and the compliance of the
orthopedic fixator to minimize motion of the skeletal elements
during a first phase of healing, increasing the adjustable
compliance of the fixator in at least one preferred direction to a
higher level during a second phase of healing, and optionally
further increasing the compliance of the fixator in at least one
preferred direction during subsequent phases of the healing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The drawings constitute a part of this specification and
include exemplary embodiments to the invention, which may be
embodied in various forms. It is to be understood that in some
instances various aspects of the invention may be shown exaggerated
or enlarged to facilitate an understanding of the invention.
[0023] FIG. 1 is a perspective view of a prior art
six-degree-of-freedom fixator device.
[0024] FIG. 2A is an elevational view of a prior art adjustable
effective length strut.
[0025] FIG. 2B is a cross sectional view of a prior art adjustable
effective length strut.
[0026] FIG. 3 is a schematic diagram illustrating a preloading
operation used in an embodiment of a fixator device of the
invention.
[0027] FIG. 4A is a schematic diagram illustrating use of a tensile
element to provide compressive preloading of adjustable effective
length struts used in an embodiment of the invention.
[0028] FIG. 4B is a schematic diagram illustrating use of a
compressive element to provide tensile preloading of adjustable
effective length struts used in an embodiment of the invention.
[0029] FIG. 5 is a schematic diagram illustrating the operation of
a preloading strut having adjustable minimum length.
[0030] FIG. 6 is a schematic diagram illustrating the sequential
process of adjusting the length of two adjustable effective length
struts and one adjustable minimum length preload strut.
[0031] FIG. 7A is an exploded view of an adjustable compliance
attachment portion used in an embodiment of the invention.
[0032] FIG. 7B is an assembled perspective view of the adjustable
compliance portion of FIG. 7A.
[0033] FIG. 7C is a top view of a portion of the adjustable
compliance attachment portion if FIG. 7B with the top spring
removed for clarity.
[0034] FIG. 7D is a section taken through line 7D-7D of FIG. 7C
with the top spring included.
[0035] FIG. 7E is a section taken through line 7E-7E of FIG. 7C
with the top spring included.
[0036] FIG. 8A is a perspective view of an adjustable parallelogram
spring portion of an embodiment of the invention with a portion of
the outer ring removed for clarity.
[0037] FIG. 8B is a perspective view of an adjustable position
clamp portion used in an embodiment of the invention.
[0038] FIG. 9 is a schematic diagram illustrating one embodiment of
a sequential process of adjusting the structural compliance.
[0039] FIG. 10 is a perspective view of an alternative arrangement
of adjustable position clamps to control the local structural
compliance.
[0040] FIG. 11A is a perspective view of an embodiment of the
invention adjusted to provide non-axial effective compliance.
[0041] FIG. 11B is a schematic diagram of a portion of an
embodiment of the invention illustrating the remote center of
rotation and non-axial effective compliance which is
achievable.
[0042] FIG. 12 is a perspective view of one embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Detailed descriptions of the preferred embodiment are
provided herein. It is to be understood, however, that the present
invention may be embodied in various forms. Therefore, specific
details disclosed herein are not to be interpreted as limiting, but
rather as a basis for the claims and as a representative basis for
teaching one skilled in the art to employ the present invention in
virtually any appropriately detailed system, structure or manner.
In the various views of the drawings, like reference characters
designate like or similar parts.
[0044] The improved orthopedic fixator device of the present
invention benefits from two concepts that can be applied
individually or in conjunction. The first concept focuses on the
pre-loading of a fixator device to reduce backlash and is
illustrated generally in connection with FIGS. 1-6. The second
concept focuses on a benefit obtained through adjustable compliance
and is illustrated generally in connection with FIGS. 7A-8B. The
combination of such concepts is shown and described in connection
with FIGS. 7A-12 that shows an improved fixator device subject to
pre-loading and having adjustable compliance.
[0045] In addition, the improved orthopedic fixator device of the
present invention is attached to human anatomy in a similar manner
that is known in the art with other orthopedic fixator devices, and
such attachment will therefore not be described herein in
detail.
Pre-Loading
[0046] Turning first to FIG. 1, there is shown a
six-degree-of-freedom external fixator known as the Taylor Spatial
Frame, which is of the general type known as a "Stewart Platform".
In this device, a lower frame 1 is provided to accept mounting
features such as Ilizarov wires or half-pins which connect to a
first bone element (not shown). There is also shown an upper frame
2, which similarly provides a mounting means for attachment to a
second bone element (not shown). The relative position of the lower
frame 1 and the upper frame 2 is determined by six adjustable
effective length struts 3, which are generally rotatably attached
to the lower frame 1 and upper frame 2 by a series of shoulder
screws 4.
[0047] The adjustable effective length struts 3 are shown more
clearly in FIG. 2A, where it can be seen that each strut 3 includes
a dual-axis pivot or "universal joint" 122 at each end. The
combination of the dual-axis pivot in the universal joint 122 and
the rotating shoulder screw 4 creates a single effective pivot
point for all three angular rotations. One of these universal
joints 122 is mounted to a threaded rod 124, while the other
universal joint 122 is mounted to the end of a cylinder 126, which
surrounds at least part of the threaded rod 124. A rotating
threaded collar 128 is threaded onto the rod 124, and is rotatably
mounted to the end of the cylinder 126 using a retaining ring 129
as shown in the cross-sectional view of FIG. 2B. By rotating the
threaded collar 128, the effective length "x" of the strut can be
adjusted to any value between some minimum length and some maximum
length. When adjusted to a given length, the adjustable effective
length strut 128 will resist both forces trying to shorten it and
forces trying to lengthen it. As a result, the position of upper
frame 2, relative to the lower frame 1, can be approximately fixed
in all six degrees of freedom (i.e., three translations and three
rotations) by independently adjusting the lengths of the six struts
3 connecting upper frame 2 to lower frame 1.
[0048] While this structure provides a reasonably effective means
of positioning frame 2 relative to frame 1, the accuracy and
precision of this positioning is limited by several mechanical
clearances and manufacturing tolerances associated with the parts
of the strut 3 and its mounting to the frames 1 and 2 through the
use of shoulder screws 4. More specifically, the exact effective
length of each strut 3 can vary by an amount that depends on the
following tolerances and clearances: vertical & lateral
clearance between the shoulder screw 4 and the upper frame 2;
clearance in the two pivots of the universal joint 122 near the
upper frame 2; thread clearance between the thread rod 124 and the
rotating threaded collar 128; axial play between the threaded
collar 128 and the outer post tube 126, as allowed by clearances
around the retaining ring 129; clearance in the two pivots of the
lower universal joint 122 near the lower frame 1; and vertical
& lateral clearance between the shoulder screw 4 and the lower
frame 1.
[0049] In general, mechanical clearances and tolerances between
elements in a structure or mechanism can effectively be eliminated
by applying forces to the elements which drive them to either the
maximum or a minimum separation allowed by the tolerances or
clearances. Thus, the imprecision caused by the clearances in the
prior art fixator could be improved if all of the individual
adjustable effective length struts 3, and their mounting to frames
1 and 2, can be preloaded either in tension or compression. The
result would be the elimination of backlash, except in cases where
some additional external loading exceeds the preload force and acts
in the opposite direction. However, if individual actuators are
preloaded, then six additional preload devices would be required.
Fewer preload devices can potentially be used if each one can
preload multiple actuators, but arbitrary positioning and
orientation of the preload forces does not guarantee the preloading
of all 6 degrees-of-freedom. Also, arbitrarily positioned preload
devices, if rigid in some direction, can result in an
over-constrained kinematic chain which will prevent the independent
adjustment of the six primary actuators. Therefore, it is essential
that any preload devices be positioned properly.
[0050] FIG. 3 schematically illustrates one very advantageous
configuration where the preload forces act on the upper and lower
frame at points which are approximately midway between the pivoting
mounting points of two neighboring actuators. When so positioned,
the combination of the preload device and the two preloaded
actuators provides the same kinematic constraint as would two
"perfect" zero-backlash actuators. The instantaneous center of
motion (in the plane of the actuators) for a segment of the upper
frame is at the intersection of the actuator axes. If a preload
device also acts through this center of motion and also lies in the
same plane as the actuators, then it can be pivotably attached to
the upper and lower frames without adding to the kinematic
constraints. Furthermore, if the preload axis approximately bisects
the actuator axes, then the preload force will be evenly
distributed across the two actuators. This example assumes the
actuators and frame segments all lie in a common plane, but the
same basic conclusion holds in three dimensions as well; a third
link mounted at a fixed fraction of the distance between the
end-points of the actuators on the upper and lower frames does not
add a kinematic constraint, but can be used to preload the two
actuators. In practice, it is not essential that the preload device
be positioned exactly along the ideal axis.
[0051] FIG. 4A schematically illustrates an embodiment of a fixator
device of the invention and one way in which a single preload
element can preload two adjustable length struts. Arrows 14
illustrate forces created by a single tension preload element, such
as a spring or other elastic device, acting on a lower frame 10 and
an upper frame 20 at points which are approximately midway between
the mounting points where two adjustable struts 12 are mounted to
the lower frame 10 and the upper frame 20. It will be appreciated
to those skilled in the art that forces 14 created by a tension
element will create compression forces, illustrated with arrows 15,
within the neighboring adjustable length struts 12.
[0052] Conversely, FIG. 4B schematically illustrates a single
preload element where preload forces indicated by arrows 16 are
generated by a preload element that is under compressive loading.
As before, these forces act on the lower frame 10 and the upper
frame 20 at points which are approximately midway between the
mounting points where two adjustable struts 12 are mounted to the
lower frame 10 and the upper frame 20. It will again be appreciated
to those skilled in the art that forces 16 created by a compressed
preload element will create reaction tensile forces, illustrated by
arrows 17, within the neighboring adjustable length struts 12.
[0053] While it might be considered that the preload forces could
be generated using another adjustable length strut 12, such an
approach would result in an over-constrained mechanism whereby the
adjustment of the three struts will be coupled, thus preventing any
large independent adjustment of any single strut and making overall
frame adjustment very difficult. Another possible approach might be
to use highly compliant spring elements to generate the preload
forces. While this could work in principle, and while it could
theoretically be used to create preload forces that never have to
be adjusted when the frame is adjusted, such an approach introduces
an additional risk. Specifically, if a highly stressed elastic
element is used to create the preload forces, a structural failure
(or intentional or accidental removal) of any one of the adjustable
struts would result in a catastrophic failure where the high
elastic preload element would generate very large displacements of
the frame. Such displacements could be strong enough and large
enough to cause significant injury, and therefore should be avoided
if at all possible. What is needed, therefore, is a convenient
means to generate preload forces which act only over a short
distance, and which do not interfere too much with the easy and
independent adjustment of the adjustable effective length
struts.
[0054] In accordance with one embodiment of the present invention,
FIG. 5 illustrates how the need for simultaneous adjustment of the
preload member and the actuator can be avoided by using a preload
strut 18 which is similar to the adjustable length struts 12, but
has been modified so that it can only be loaded in compression.
Preload strut 18 can easily be created by eliminating the retaining
ring 129 (FIG. 2B) which normally holds the threaded collar 128 in
a relatively fixed axial position relative to the outer strut tube
126 in adjustable struts 12. Without any axial retention means, the
threaded collar 186 of preload strut 18 will be free to move
(together with the threaded rod 184) axially away from the end of
the outer strut tube 182. In the illustrated embodiment, the
preload strut 18 can be adjusted to have any desired minimum length
(within some range) and can accommodate compression (i.e.,
shortening) forces, but is free to extend out to a maximum length
with essentially no tension forces. In the particular preload
device 18 illustrated in FIG. 5, when the rotating nut "N" (also
collar 186) is "jammed" up against the body B (also tube 182), then
the preload actuator will extend slightly until any clearance or
backlash is removed from the adjustable struts 12 on either side.
But if the adjuster nut N (186) is simply backed away a few turns,
then strut 12 on either side can be adjusted independently, just as
when there was no preload actuator 18 present.
[0055] In the illustrated embodiment, the amount of preload force
is generally proportional to the torque which is applied to tighten
nut 186 against body tube 182. It will be appreciated to those
skilled in the art, however, that there are many ways which could
be used to limit the maximum preload force. One such approach for
limiting the maximum preload force would be the use of a one-way
torque-limiting slip clutch on nut N, similar to the approach used
on modern automotive gasoline caps to limit the available
tightening torque without limiting the torque available to remove
or loosen the threaded cap.
[0056] The assembly shown in FIG. 5 is meant to illustrate how
preload element 18 can be used to remove the clearances from two
adjacent adjustable struts 12. As previously noted, either of the
adjacent struts 12 can be independently adjusted whenever the
preload adjustment nut 186 is adjusted away from tube 182 of
preload strut 18. An important additional feature of the
illustrated assembly results whenever the ends of preload strut 18
are positioned at, or very near, the midpoints between the ends of
neighboring struts 12. In such a preferred configuration, where the
preload strut between two adjustable struts is positioned at the
convergence of a pair of adjustable effective length struts so that
it does not significantly add to the kinematic constraints afforded
by the struts 12 on the allowable orientation of the upper frame 20
relative to the lower frame 10, the four adjustable struts 12 which
are not the immediate neighbors of preload strut 18 can be
independently adjusted even when the preload strut 18 is providing
preload to the two neighboring struts 12.
[0057] FIG. 6 illustrates a fully populated version of a fixator
device of a preferred embodiment of the present invention, where
three preload struts 18 are used to preload all six adjustable
struts 12. Each individual preload strut 18 is positioned between a
pair of adjustable struts 12, and provides preload to that pair of
struts. It should be apparent to those skilled in the art, that for
small adjustments of the strut 12 lengths when the preload struts
18 are positioned sufficiently close to the ideal locations
described previously in FIG. 3, each preload strut 18 only prevents
the shortening of an immediately neighboring adjustable strut.
Lengthening of a neighboring strut will have the effect of removing
the preload, and can be done without first loosening the preload
device. Furthermore, lengthening or shortening of any non-neighbor
struts will not be significantly affected by the preload generated
by a non-neighboring preload strut. Therefore, it is possible to
maintain maximum frame stability while making desired frame
adjustments by only loosening one preload strut at a time and
adjusting only the neighboring struts 12 before re-applying the
preload from strut 18. In other words, the actuation of a preload
strut 18 constrains independent adjustment of adjacent adjustable
effective length struts 12 while not substantially constraining
independent adjustment of non-adjacent adjustable effective length
struts 12. The preferred adjustment operation for maximum stability
(i.e., minimum positional uncertainty during adjustment) is
illustrated schematically in FIG. 6 and can be described as the
following:
[0058] Step S1) Release one of the preload nuts on one of the three
preload struts 18.
[0059] Step S2) Shorten or lengthen the strut 12 on one side of the
preload strut 18 to the desired length.
[0060] Step S3) Shorten or lengthen the strut 12 on the opposite
side of the preload strut 18 to its desired length.
[0061] Step S4) Tighten the preload nut on the selected preload
strut 18 to preload the two neighboring struts 12 in their new
positions.
[0062] Repeat steps S1-S4 for each of the remaining two "triplets"
comprising one preload strut 18 and two adjacent adjustable
effective length struts 12.
[0063] In some cases, it may not be possible to position the ends
of the preload struts 18 near the ideal points described in FIG. 3,
and the resulting non-ideal positioning of the preload struts may
create kinematic binding which makes it difficult to adjust some
strut lengths even when the neighboring preload strut is
disengaged. In those cases, or simply when it may be more
convenient to loosen all preload struts first, an alternative
adjustment process would be to release the preload nuts 186 on all
three preload struts 18, then shorten or lengthen each of the six
struts 12 to achieve the desired length, and then re-tighten the
preload nuts 186 on all three preload struts 18.
Adjustable Compliance
[0064] The medical literature (e.g., Wheeless' Textbook of
Orthopaedics, available online at www.wheelessonline.com) indicates
that bone fracture healing and bone regeneration is significantly
affected by the level of mechanical rigidity that is provided by
fixation devices during the healing process. More specifically, it
is generally believed that the optimal level of fixation rigidity
varies during the fracture healing process, with maximum rigidity
(i.e., minimum compliance) generally being most appropriate during
the initial primary healing or callous formation phases, and with
progressively lower rigidity (i.e., higher compliance) being most
appropriate during the later callous remodeling phase when load
sharing between the bone and the frame is required. Current
orthopedic fixation devices, such as the Taylor Spatial Frame, have
manufacturing tolerances that significantly limit the maximum
stiffness that can be achieved for small displacements, and they
provide no controllable means for reducing the stiffness (i.e.,
increasing the compliance) during later healing stages.
[0065] The increased maximum stiffness which is desirable from an
orthopedic healing perspective has been provided in the previously
described embodiments via the incorporation of pre-loading elements
18 to substantially remove the effect of manufacturing tolerances
and necessary clearances in an assembly comprising a plurality of
adjustable effective length struts 12 which define the spatial
orientation of an upper frame 20 relative to a lower frame 10. In
accordance with additional embodiments described herein, we now
teach methods of adjustably decreasing the stiffness (i.e.,
increasing the compliance) of the orthopedic fixator to values
below that of the maximum stiffness achieved using the structure
shown in FIG. 6.
[0066] In FIG. 6, the upper frame 20 is connected directly to the
adjustable length struts 12 as well as to mounting structures
attached to a bone element (not shown). In accordance with an
additional embodiment of the invention, there is shown in FIGS.
7A-7E, where FIG. 7A is an exploded view, FIG. 7B is an assembled
view, FIG. 7C is a top view and FIGS. 7D and 7E are cross-sectional
views, an additional preferred embodiment of a fixator device
comprising an upper plate 20a (sometimes referred to as upper ring
20a) that is flexibly attached to an inner upper plate 30
(sometimes referred to as inner upper ring or inner ring 30) which
is, in turn, connected to mounting structures attached to a bone
element (not shown). In the particular device illustrated, the
flexible or compliant attachment between the upper plate 20a forms
an outer ring which surrounds the inner upper plate 30. One or more
spring plates 40, 42 are clamped to the upper plate 20a using bolts
41 and nuts 43, or any other suitable means as would be known to
those skilled in the art. Inner ring 30 has radially extending wing
segments 33 which support clamp elements 50. Clamp elements 50 are
designed to clamp a region of the spring plate(s) 40, 42 in
addition to the radial extension 33 of the inner ring 30. FIGS.
7C-7E illustrate the engagement of the rings 20a, 30 with the
plates 40, 42 along various sections of the rings 20a, 30. The
upper ring 40 is not shown in FIG. 7C only for purposes of clarity,
even though it is represented in the sectional views FIGS. 7D and
7E. Furthermore, the inner and outer rings 20a, 30 are concentric
in the present embodiment, although they may be positioned in a
non-concentric manner if desired.
[0067] In addition, while the upper plate 20a is shown attached to
the lower frame 10 as shown more clearly in FIG. 8A among others,
it will be appreciated that a reverse construction could be
contemplated where the inner ring or plate 30 is attached to the
lower frame 10 and the outer ring 20a is compliantly attached to
the inner ring 30, where the structural features and functions of
the outer ring 20a and inner ring 30 as described herein are
essentially reversed. Alternatively, for example, the outer ring
20a could still be attached to the lower frame 10, but the outer
ring 20a has inwardly projecting radial features and the clamps 50
fix the spring plates 40, 42 to the outer ring 20a. Thus, instead
of the inner ring 30 having radially extending wing segments 33
which support clamp elements 50, with clamp elements 50 designed to
clamp a region of the spring plate(s) 40, 42 in addition to the
radial extension 33 of the inner ring 30, the outer ring 20a would
have inwardly extending radial extending wing segments which
support clamp elements 50, and the clamp elements 50 would be
designed to clamp a region of the spring plate(s) 40, 42 in
addition to the radial extension of the outer ring 20a. So, not
only can the inner and outer rings 30, 20a respectively be reversed
with respect to which one is connected to the lower frame 10, but
also the selection of which ring is fixed to the spring plates 40,
42 and which ring is clamped to the spring plates can also be
reversed, regardless of which ring is connected to the lower frame
10. Thus, the structural embodiments illustrated herein are not
meant to be interpreted in a limiting sense as set forth below.
[0068] The adjustable compliance provided through the use of inner
and outer rings as illustrated herein, for example, can be utilized
on a fixator device having pre-load elements 18 described above and
as shown in FIG. 12. Alternatively, the prior art design of FIG. 1
without preloading could be further incorporated with adjustable
compliance features as described herein for purposes of improving
the benefits and features of the prior art fixator device with the
benefits of adjustable compliance. Thus, the benefits and features
of preloading and adjustable compliance can be realized
individually or jointly. FIG. 12 illustrates one embodiment
comprising multiple benefits as described herein, including both
preloading and adjustable compliance. For purposes of explanation,
the embodiments of FIGS. 7A-11B will be representative of the
fixator device shown in FIG. 12, it being understood that other
configurations are possible.
[0069] An important aspect of the illustrated embodiment is the
ability for the multiple clamps 50 to be positioned at various
points around the periphery of the radial extensions 33 of the
inner ring 30, such that the attachment of the spring elements 40,
42 preferably occurs along a circumference defined along the outer
ring 20a. If the clamps are positioned midway along the free length
of spring elements 40, 42, then two equal length flexural spring
regions are formed on opposite sides of the clamp 50. If, however,
the clamp is positioned very close to a point where the spring
element 40 or 42 is mounted or clamped to plate or ring 20a, then
the length of the unsupported spring element to one side of the
clamp 50 is smaller than the unsupported length of spring element
to the opposite side of the clamp 50. While the sum of the two
unsupported spring lengths is fixed by the geometry of the spring
plate 40 or 42, the position of the clamp 50 determines how that
total length is divided into two complementary segment lengths.
[0070] As is known to those skilled in the art, the bending
stiffness of a plate spring is proportional to the cube of the
length of the spring. The result of this non-linear relationship
between stiffness and length means that two springs of equal length
will have a lower stiffness than one shorter and one longer spring
that add up to the same total length. In other words, two springs
which are each two centimeters long each will combine to have a
lower stiffness than one centimeter long spring together with one
three centimeter long spring. As one spring approaches zero length
and the other spring grows by the same amount, the total stiffness
will continue to increase, up to the theoretical limit where one
spring has zero length and nearly infinite stiffness. Furthermore,
this nonlinear force-displacement is such that a displacement of
the inner ring 30 relative to the outer ring 20a increases by less
than a factor of two when an external force applied to the
compliant attachment increases by a factor of two.
[0071] In accordance with the present invention, the effective
stiffness of the elastic mounting between plates 20a and 30 can be
adjusted over a wide range of values simply by changing the
position of the clamp elements 50. When the clamps 50 are
positioned midway along the length of spring plates 40, 42 (in
between the points where spring plates 40, 42 are mounted to outer
ring 20a), as shown in FIG. 7B, then the total effective stiffness
of the elastic mounting between plates 20a and 30 is minimized. If
one or more of the clamps 50 is moved closer to any of the points
where spring plates 40, 42 are attached to outer ring 20a, then the
stiffness increases. And if the clamps 50 are placed immediately
next to the locations where spring plates 40, 42 are attached to
outer ring 20a, then the stiffness is maximized (i.e., the
compliance is minimized).
[0072] Having observed the details of the effect of dividing a
fixed length of spring into two complementary spring lengths,
attention may now be given to the details of the spring plates
themselves and the clamps elements 50 used to fix a region of the
spring plate 40 to the radial extension 33 of the inner ring 30.
FIG. 8A shows a close-up view of the outer ring 20a, a segment of
which is removed for clarity, the inner ring 30, an upper spring
plate 40, and a lower spring plate 42, along with the movable clamp
elements 50. In accordance with one aspect of the invention, both a
lower spring plate 42 and an upper spring plate 40 can be used to
flexibly attach outer plate 20a to inner plate 30. As will be
understood to one skilled in the art, the use of two parallel
springs mounted some reasonable distance apart, but clamped at the
same position so that they have the same free length, will create a
parallelogram spring assembly which has relatively low stiffness
against vertical translation and against rotation around the axis
of the springs, but provides high stiffness against translation in
the plane of the rings and rotation around the axes orthogonal to
the spring axis. Since, in the embodiment shown, the three sets of
springs positioned around the ring 20a have different axes, the
combination of springs provides compliance (low stiffness)
primarily for vertical translation only.
[0073] The clamping of a portion of the spring plates 40 and 42 can
be accomplished in many ways. It is preferable, however, for the
clamp assembly to be easily repositioned and to have a minimum
number of separate elements which have to be positioned properly.
This is accomplished in one embodiment of the present case as
illustrated in FIG. 8B, which shows a single clamp element 50,
comprising integrally formed spacer elements 502, 504, and 506
attached to the main body of clamp 50 through thin flexible flexure
elements 510. The spacer elements fill the gaps between the spring
plates 40 and 42 and the radial wing extension 33 of the inner ring
30, so that actuation of a single set-screw 52 or the like can be
used to clamp a region of both spring plates 40 and 42 firmly
against the radial wing extension 33 of inner ring 30. Thus, in the
present embodiment, the effective complementary lengths of the dual
parallelogram springs which are formed on each side of clamp 50 can
be easily adjusted by loosening screw 52, sliding the clamp 50 to a
new circumferential position around the inside perimeter of ring
20a, and then re-tightening the screw 52. By always having two
clamps tightened when one is loosed, the inner ring 30 will be
prevented from rotating or shifting laterally. In addition,
alignment markings or scales of various types can be provided on
one or more of elements 20a, 30, 40, or 42 to visually indicate
specific locations for clamps 50 and (optionally) the resulting
compliance associated with those clamp locations.
[0074] In the embodiment illustrated in FIG. 9, the axial
compliance of the elastic mounting between outer plate 20a (a
portion of which is removed for clarity) and inner plate 30 can be
adjusted without ever completely decoupling plates 20a and 30,
through the following recommended adjustment sequences. The first
sequence illustrated in FIG. 9 by reference number 51a comprises
loosening the clamp screw 52 on the first of three clamps 50,
sliding the first clamp 50 to the desired location to provide the
appropriate stiffness and then tightening the clamp screw 52 on the
first clamp 50. The second sequence illustrated in FIG. 9 by
reference number 51b comprises loosening the clamp screw 52 on the
second of three clamps 50, sliding the second clamp 50 to the
desired location to provide the appropriate stiffness and
tightening the clamp screw 52 on the second clamp 50. The third
sequence illustrated in FIG. 9 by reference number 51c comprises
loosening the clamp screw 52 on the third of three clamps 50,
sliding the third clamp 50 to the desired location to provide the
appropriate stiffness and tightening the clamp screw 52 on the
third clamp 50. Of course, the order of sequences 51a-51c can be
modified as desired.
[0075] Various details of the mounting and clamping of the springs
to the outer ring 20a can be changed without departing from the
spirit of the invention, with the addition of outer clamping plates
being one possible modification to improve the mounting stiffness.
The clamps are shown as one-piece elements fabricated using a
wire-EDM process to form spacers and a force spreading block that
are free to float vertically by a small amount to account for
tolerances, but which are integral to the clamp and thus forced to
translate with the block. Other clamp designs and configurations
are possible, including, but not limited to, those with free
floating spacer elements, etc., or spacers which are constrained by
other means (e.g., pins or other geometry) to translate with the
clamp block during installation/adjustment.
[0076] FIG. 10 illustrates another embodiment of the invention
where a larger number of clamping elements 50 is used. In the
particular embodiment illustrated, each of the three
circumferential regions of spring plates 40 and 42 are clamped with
two clamps 50. In this case, the free length of the dual
parallelogram springs can be equal. These dual clamps are shown in
various potential desirable configurations, including: a
minimum-stiffness (i.e., maximum compliance) case 53a where the two
clamps 50 are both close to the middle of the flexing portion of
spring plates 40 and 42; an intermediate stiffness case 53b where
the two clamps are symmetrically positioned some distance away from
both the center of, and the mounting region of, the flexing portion
of spring plates 40 and 42; and the maximum stiffness case 53c
where both clamps 50 are positioned very near the location where
spring plates 40 and 42 are attached to outer ring 20a.
[0077] In some medical circumstances, it may be desirable to create
non-axial compliance, i.e., a remote center-of-rotation, between
the inner ring 30 and the outer ring 20a. In accordance with the
present invention, such non-axial compliance can be created as
shown in FIG. 11A, where the axial stiffness of the three spring
regions positioned around the periphery of the circular ring plates
is intentionally adjusted to be unequal. By positioning some clamps
for higher stiffness (clamp pair 53d) than other clamps (clamp pair
53e), it is possible to modulate the effective compliance to create
a remote center of motion. This could be useful, for example, if
the axis of the bone is perpendicular to the plane of the mounting
ring, but the fracture plane is not orthogonal to the bone axis. In
the example shown in FIG. 11A, two clamps (53d) are used to create
high stiffness in one of the three spring sectors, while the clamp
positions in the other two sectors are adjusted to provide lower
stiffness. The resulting non-uniform axial stiffness of the ring
support points will result in a rocking motion under loading, and
this combination of axial deflection plus rocking may better resist
separation of an angled fracture plane. As illustrated in FIG. 11A,
two clamps (53d) are positioned so as to create relatively short
spring elements at the right-hand side of the spring plates so that
the axial stiffness at that point will be higher than that provided
by the single, centrally located clamps (53e) at the other two
positions approximately one hundred and twenty degrees around the
ring. The net effect of this non-uniform spring stiffness is to
create a non-axial compliance and a remote-center-of-rotation axis
60 effect by which the inner ring 30 will not respond to an axial
load with direct axial motion, but will instead, respond to an
axial load with a combination of axial motion plus some tilting
rotation about an axis 60 somewhere to the right of the assembly,
as illustrated schematically in FIG. 11B with the movement of the
inner ring 30 between a first position 30a and a second position
30b. It should be appreciated that this movement is illustrated in
FIG. 11B to a greatly exaggerated extent, where in practice no
portion of any of the inner ring extensions should translate
vertically by more than the provided clearance, which is +/-2.5
millimeters in this embodiment.
[0078] An additional feature of the invention is the optional
ability to provide non-linear compliance between the upper inner
ring 30 and the upper outer ring 20a. A spring is said to be linear
if a doubling of the external force produces a doubling of the
deflection. For best bone healing, especially during secondary
healing and callus consolidation, it is important that the healing
bone experience some external loading, but it is also important
that the healing bone joint be protected from excessive external
loads. A non-linear force deflection curve is thus desirable,
whereby a larger fraction of small external loads is transferred to
the bone, but only a smaller fraction of very large external loads
is transferred to the bone. In the preferred construction shown in
FIGS. 7-12, such a non-linear force deflection curve is naturally
produced as a result of the use of spring elements where both ends
of the springs are fixed. As is known by those skilled in the art,
a cantilever spring element produces a largely linear restoring
force in response to an applied deflection, but a spring plate or
membrane which is held at both ends produces a non-linear restoring
force in response to an applied deflection somewhere along its
length. More specifically, the equation defining the
force-deflection curve contains a term proportional to the cube of
the deflection. Thus, the spring gets progressively stiffer as the
deflection increases, which is precisely what is desired to help
protect the bone from large external forces while still
transmitting a significant fraction of small external forces.
[0079] The same analysis holds true for the parallelogram spring
construction created through the use of both an upper spring plate
40 and a lower spring plate 42. While a single parallelogram
flexure with a free end would behave linearly (for reasonably small
deflections), the fixed mounting of the ends of the two
parallelograms on either side of the clamp creates a membrane
spring effect, and thus the stiffness will increase as the
deflection is increased. As noted previously, membrane
force-deflection expressions have a cubic term in the
force-deflection curve and therefore, this design naturally
provides lower stiffness for small loads, and higher stiffness for
higher loads. If nonlinear stiffness is not desired for some
reason, the spring plates described in this invention can be
modified so that one end is free. In that case, the membrane forces
will be eliminated and the spring will generate a linear
force-deflection profile.
[0080] In a further embodiment, each of the spring plates 40 and 42
can be replaced by a plurality of plates. The ability to use a
larger number of thinner springs, instead of a smaller number of
thicker springs, for example, enables a spring designer skilled in
the art to provide a wide range of stiffnesses, while also
accommodating a desired amount of deflection with an acceptable
amount of material stress. The number and thickness of the spring
rings used determines the available stiffness range for each spring
section, and the position of the clamps 50 determines the selected
level of compliance within that available range.
[0081] Yet another important safety feature of the invention is the
ability to limit the maximum motion of the inner ring 30, and thus
to limit the maximum motion of the attached bone segment (not
shown). In accordance with the construction of the embodiment
illustrated in FIGS. 7-12, the motion of the inner ring 30 is
naturally limited in all directions, regardless of the stiffness of
the springs and the clamp position. The inner ring 30 is
constrained by clearances with the outer ring 20a against large
radial motion and circumferential rotation. Vertically, each radial
extension of the inner ring 30 is independently limited by the
distance between the radial extension surfaces and the top and
bottom surface of the outer ring 20a to which the springs are
mounted. These clearances (preferably on the order of 2.5
millimeters each in the embodiment shown) can be unequal if
desired, so extension motion can be limited to a different value
than compression motion. In any case, vertical motion larger than
the clearance would require shearing one or more springs near their
connection to the outer ring 20a.
[0082] An important set of safety features of the invention is the
provision of mechanical redundancy in all critical areas, which
prevents any single failure of a mechanical component from leading
to a catastrophic failure of support. If a preload strut 18 fails,
the external loads will be supported by the adjustable effective
length struts 12, although with some loss of positioning precision
and rigidity. If an adjustable effective length strut 12 fails,
external compressive loads will be supported by neighboring preload
struts 18. Although a large extensional load could still cause a
large deflection at the bone, even this theoretical failure mode
could be addressed by a simple modification of the preload strut 18
so that the adjustable nut 186 is constrained by a retaining ring
in an overly wide slot such that retaining nut 186 can only move
axially some reasonable maximum distance, preferably on the order
of a few millimeters, relative to the outer tube 182 of the preload
strut 18.
[0083] As a further example of mechanical redundancy which prevents
single points of failure from causing potentially catastrophic
failure to maintain acceptable bone positions, each spring plate
preferably comprises multiple independent flexing regions and
multiple independent points of fixation to both the outer ring 20a
and the inner ring 30. If the spring fails at any one point, the
motion of the system is still limited by other spring regions and
by the mechanical limit stops created by the nesting of the radial
extensions 33 of the inner ring 30, each of which is confined
inside the outer ring 20a, underneath the upper spring 40, and
above the lower spring 42. Similarly, if a clamp element 50 or a
clamping screw 52 fails, the inner ring will still be supported and
constrained to prevent excessive motion.
[0084] One process for orthopedic fixation of two skeletal elements
during healing comprises fixing a position of a first skeletal
element relative to a second skeletal element using an orthopedic
fixator with adjustable compliance as discussed above, adjusting
the position and the compliance of the orthopedic fixator to
minimize motion of the skeletal elements during a first phase of
healing, and increasing the adjustable compliance of the fixator in
at least one direction during a second phase of healing. A further
process comprises increasing the adjustable compliance of the
fixator in at least one direction after the second phase of
healing.
[0085] Having thus described several embodiments of the present
invention, it will be appreciated that such embodiments represent
an orthopedic fixation solution having many or all of the following
beneficial characteristics:
[0086] Precision--the ability to adjust the relative position of
two bone segments in six degrees of freedom to within a very small
degree of error.
[0087] High Maximum Stiffness--the ability to maintain any adjusted
position with very little motion occurring in any direction as a
result of external forces.
[0088] Adjustable Compliance--the ability to controllably reduce
the effective fixation stiffness below a maximum level during
select healing phases. Although step-wise stiffness control may be
acceptable, a preferred solution would allow for continuous
stiffness adjustment between some maximum and minimum level.
[0089] Anisotropic Compliance--the ability to provide higher
compliance (i.e., lower stiffness) along some direction (typically
the bone axis or perhaps the direction perpendicular to a fracture
plane) than in all other degrees of freedom.
[0090] Nonlinear force-deflection curve--for any overall stiffness
level selected, the provision of a lower incremental stiffness at
smaller loads can enable significant load sharing to the bone at
low loads, while a higher stiffness at larger deflections ensures
that a larger fraction of abnormally high input forces are
transferred to the fixation frame instead of the bone.
[0091] Motion limit stops--the incorporation of physical
constraints designed to limit total possible bone motion to a safe
maximum level regardless of the load applied and the frame
stiffness adjustment. Most preferably, it should also be possible
to limit the maximum deflection in one direction independently of
the opposite direction, so that maximum bone extension can be
limited to a different value than maximum bone compression, if
desired.
[0092] Structural redundancy--the design of structural elements and
the duplication of key parts such that failure of any single
component will not result in excessive displacement of the
bone.
[0093] While the invention has been described in connection with a
preferred embodiment, it is not intended to limit the scope of the
invention to the particular form set forth, but on the contrary, it
is intended to cover such alternatives, modifications, and
equivalents as may be included within the spirit and scope of the
invention as defined by the appended claims.
* * * * *
References